The invention relates generally to printed circuit boards, and deals more particularly with a method for fabricating printed circuit boards having cavities.
Printed circuit boards (PCBs) are well known and comprise one or more layers of dielectric material with metallizations on one or both surfaces of each layer. The dielectric layers may be made from glass-reinforced polyimide, Teflon (a trademark of E.I. duPont de Nemours) or epoxy. The metallizations comprise conductors, pads, surface lands, ground planes and power planes. Layers of different dielectric materials can be laminated together into the same board.
Some boards require cavities in one or more layers of the PCB. The cavity may be entirely plated and grounded and used to house a power amplifier to reduce stray emissions. The leads of the chip may pass through the plating at the bottom of the cavity and pass through the lower layers for attachment at the opposite face of the PCB. Alternately, the cavity can house a wire-bond chip oriented such that the connecting pads are face up.
It was known to fabricate a PCB with a cavity as follows. A window is cut-out in an epoxy core layer and a matching window is cut-out in an epoxy-glass sticker sheet. The sticker sheet may comprise "Pre-Preg" and is formed as follows. A fiberglass cloth, woven Kevlar(tm) or other woven or non-woven glass cloth is dipped into a solution comprising epoxy and a solvent/thinner. Then the cloth is pressed through rollers to squeeze out excess epoxy and thinner. Next, the cloth is partially dried to remove the solvent and partially cure the epoxy. The degree to which the epoxy remains uncured determines the degree to which the pre-preg will flow when subsequently laminated.
The epoxy core layer and epoxy-glass sticker sheet are laid on a solid Teflon-glass core layer with the sticker sheet in the middle such that the windows in the epoxy core layer and epoxy-glass sticker sheet are registered with each other. Then, the laminate is heated to filly cure the epoxy-glass sticker sheet and thereby bond the epoxy core layer to the Teflon-glass core.
However, a problem has resulted in that a "high-flow" type of epoxy-glass sticker sheet (i.e. relatively uncured before lamination) seeps into the cavity in the epoxy core layer during lamination thereby covering the metallization on the Teflon-glass core layer. A "no-flow" or "low flow" type of epoxy-glass sticker sheet will reduce such seepage but is much more expensive than the "high-flow" type and tends to trap air during the heating/lamination process. Also, the windows in the low-flow sticker sheet must be closely tailored to the windows in the epoxy core layer before lamination, for example a 0.02" recess back from each side of the window to permit the epoxy-glass sticker material to flow to the edges of the window in the epoxy core layer. Even with this precision, the low-flow epoxy-glass sticker material does not form a good seal at the base perimeter of the cavity due in part to the substantially pre-cured nature of the epoxy. Ideally, there would be a continuous fillet of the epoxy-glass sticker material at the base perimeter of the cavity to form a good seal between the epoxy core layer and the Teflon-glass core layer and not cover the bottom of the cavity.
U.S. Pat. No. 4,636,275 discloses an elastic bladder which is pressed by a fluid into a cavity in epoxy glass layers to seal the layers while adhesive between the layers is cured.
U.S. Pat. Nos. 4,643,935 and 4,680,075 disclose a thermo-plastic plug which is initially shaped by pressing a thermo-plastic blank into a dummy cavity. Then, the plug is inserted into a similarly-shaped cavity in a real multi-layer board, and the multi-layer board is laminated. The plug deters seepage.
While thermoplastic materials are generally effective to plug cavities, the thermoplastic materials become softer and softer with rising temperature, i.e. they melt. Some lamination processes require such high temperature that the thermoplastic material may be too soft to effectively dam a high flow pre-preg sticker sheet.
A general object of the present invention is to fabricate a printed circuit board laminate with a cavity in one or more layers in such a way that is cost effective, forms a good seal at the base perimeter of the cavity, securely bonds the layers together and does not permit adhesive from between the layers to coat the bottom of the cavity.